xna × 2796
xna-4.0 × 657
These tags mean the same. I think we can assume, that user is using the latest and last version of XNA (4.0). So when looking for xna tags, user expects to see 4.0 tags also.
There's third tag also xna3.1 with 6 questions. XNA 3.1 and 4.0 has differences, so maybe that should be left alone?
I just feel that users looking for certain tags, expect certain results. Looking for unity,xna, monogame,sdl..etc, should bring all versioned stuff, but looking for versioned tag, should return only questions tagged with that.
I am not saying that we should delete all versioned tags or anything, but these two most used xna tags are synonyms.
EDIT: There is currently 318 questions that have tagged xna and xna-4.0.